It seems OBS is dropping audio sources at random for any configuration. I've a Soundcraft SIgnature 12 MTK mixer. Input sources are randomly dropped. Dropped from recording, dropped from the stream and dropped from monitoring. Sometimes, only dropped from recording, most of the times dropped from monitoring, and occasionally dropped from the stream.
This happens all the time.
Tried it with a Tascam US16x08. Same terrible results.
I saw this problem reported here since 2014.
COME ON!!!!!!!!!
6 years have passed and the same bug is still here.
Conclusion: Change the OBS version back numer 0.25, because it's unstable as hell. It's absoluitely unworkable. It's functions as an alpha release.
OBS falls into the category super buggy toys.
It's absolutely not worth investing your time in this piece of crap.